Progress bar rework

The work on #340 is **not** finished but I'm creating this PR for an early review of the changes.

- The old `ProgressBar` is now renamed to `UploadProgressBar`. It extends wicket-extensions' UploadProgressBar and renders Bootstrap markup.
TODO: find out how to extend features from both wicket-extensions' `UploadProgressBar` and the new `ProgressBar`

- The new `ProgressBar` supports all features from http://getbootstrap.com/components/#progress (stacks, colors, stripes, animations)

Please note the following things:

- the usage of inner non-static class `Stack` that requires API usage like: `Stack stack = progressBar.new Stack(...)`. I've made it this way because it hides internals like the usage of a repeater for the stacks and the need of `#getChildId()` method. 

- `ProgressBar` can create a default `Stack` that is completely hidden from the user code by using any of the constructors with more than one parameter.

The best way to see the user API is to check `de.agilecoders.wicket.samples.pages.ComponentsPage#addProgressBars`.